Transcript

Dr Allan McInnes

Low-Earth orbits are orbits between about 200 kilometres and 2000 kilometres. Most higher orbits tend to be around 36,000 kilometres. A medium-Earth orbit would be things in between that, although there’s a range of orbits, we don’t tend to use because there are areas where we have high amounts of radiation. The Earth is surrounded by what are called the Van Allen radiation belts, and at certain altitudes, the radiation’s just too intense to be able to put a satellite there and have it last a long time.

So what we tend to find is, if we’re looking at medium-Earth orbits, things between about 15 000 kilometres and maybe 24,000 kilometres are sort of a safe region where we could put satellites and not have too much radiation. Not many satellites use medium-Earth orbits. Either we want to be in a low-Earth orbit so we can see the Earth in detail or we want to be up high. One of the things that does use medium-Earth orbit is the global positioning system or GPS. The GPS satellite constellation, which is the group of satellites, is carefully designed to make sure that any time you should be able to see at least four satellites overhead, that’s required for your GPS receiver to work. Now one way to do that would be put huge numbers of satellites up there, another way would be to make sure that the satellites are moving in a way that you can have just a few satellites up there and still be able to get four overhead at all times. And it turns out that around about 20 000 kilometres altitude works well for that.